I don't know if this behavior is intentional, or what sense it makes, but...
When I create a level with a space for terrain around my house, e.g., with a depth of -200, and then activate the generator, the generator's preview window always shows the terrain mesh to be edited only at level 0, without the actual room depth. As a result, the ratio of terrain to house doesn't match, and terraforming can only be done by feel and in "blind flight." Only after completing the generator does the terrain appear at the correct level. This makes it practically impossible to orient the terrain to the house.
Am I doing something wrong?

That is the default (and fixed) behavior. The floor thickness is not a data point for the generator, only the room circumference. The generator creates 'depth', i.e. raises selected points of the terrain grid. At the bottom right you can select to display a wire frame of the house.

The fact that the generator always uses level 0 was discussed some time ago, until then I didn't even realize that was the default behavior and I experimented a lot!
You state that the ratio of terrain to house doesn't match but I assume you mean the (start-)height of the terrain that you want at a level with elevation -200.
I depends on what you want to achieve but since you want the terrain object at a level -200 I assume you want parts of the terrain below the 0 elevation. There are tricks but it depends on what what result you want. Explain it a little more and I can give better advise.